The CD4 T cell specific protein interacts both with the HIV-encoded glycoprotein gp120 with class II MHC glycoproteins on the surface of antigen presenting cells. In the former case CD4 acts as a receptor for HIV infection, and in the latter case forms an essential component of an important cell-cell interaction in the normal immune response. The specific aim of this project is to define peptides of CD4 and of a class II MHC glycoprotein which interacts in the normal response by examining possible peptide inhibitors of this interaction and to test any compound which may inhibit the interaction of CD4 with gp120 for its ability to inhibit the CD4-class II MHC glycoprotein interaction using the same assay. Other approaches to defining the epitopes of CD4 and of the class II MHC glycoproteins involved in their interaction are described.